1. Field of the Invention
The present invention is directed to a network termination unit for connecting analog and/or digital terminal devices to a interface of an ISDN telecommunications network.
2. Discussion of Background and Relevant Information
It is anticipated that only digital subscriber terminal devices (Integrated Services Digital Network terminals, commonly referred to as ISDN terminals) will be used when the final implantation of ISDN telephone exchange networks, as described, for example, in "ISDN Das Dienstintegrierende Digitale Nachrichtennetzwerk" [ISDN--The Integrated Services Digital Communications Network] by P. Bocker, published by Springer Verlag in Berlin, 1990, is achieved. However, for the immediate future, telecommunication networks employ both conventional (i.e., analog) terminal devices and ISDN (digital) terminals.
Conventional terminal devices are connected to an S interface of the ISDN, in accordance with the ISDN specification, via an a/b interface (see, for example, page 102 of Kommunikation Mit ISDN [Communications Using the ISDN] by Dr. H. E. Martin, published by Markt & Technik Verlag, Munich, 1988, or page 29 of "ISDN Das Dienstintegrierende Digitale Nachrichtennetzwerk" [ISDN --The Integrated Services Digital Communications Network] by P. Bocker, published by Springer Verlag in Berlin, 1990). An ISDN network terminator (NT12, NT1 or NT2) must be provided between a two-wire subscriber line, connected to the central exchange, or between a U interface and the S interface. This results in a complicated setup in order to connect conventional (analog) terminal devices to the ISDN.
Three types of network terminators exist; NT1, NT2 and NT1, 2 network terminators. Network terminator i (NT1) includes functions that may be regarded as belonging to OSI (Open Systems Interconnection) layer 1; that is, functions associated with the physical and electrical termination of the ISDN on the user's premises. The network terminator 1 (NT1) may be controlled by the ISDN provider and forms a boundary to the network. This boundary isolates the user from the transmission technology of the subscriber loop and presents a new physical connector interface for user device attachment. In addition, the network terminator 1 (NT1) performs line maintenance functions, such as, for example, loop-back testing and performance monitoring.
Network terminator 2 (NT2) is an intelligent device that may include, depending on the requirement, up through OSI layer 3 functionality. Network terminator 2 (NT2) can perform switching and concentration functions.
Network terminator 1, 2 (NT12) is a single device that contains the combined functions of the network terminator 1 (NT1) and the network terminator 2 (NT2).
European Patent Application EP 0 495 423 discloses a connection device, in which two analog terminals can be economically connected to the U interface of the ISDN telecommunications network. However, an ISDN network termination unit is still required that is connected to the U interface and that has the S interface with which the digital terminals can be connected directly, and in which the analog terminals can be connected via a terminal adapter (TA) if a subscriber wishes to connect both analog and digital terminals, as shown at page 29 and FIG. 2.7 in "ISDN Das Dienstintegrierende Digitale Nachrichtennetzwerk" [ISDN--The Integrated Services Digital Communications Network] by P. Bocker, published by Springer Verlag in Berlin, 1990.